Counter-torque tail rotors are often used in helicopters. Tail rotors can be mounted adjacent to vertical fins that provide for aircraft stability. With this configuration, the helicopter rotor produces a transverse airflow. Tail rotors can be driven at high angular velocities to provide adequate aerodynamic responses. Sometimes, vortices produced by a main helicopter rotor and the tail rotor can interact to reduce the efficiency of the thrust created by the rotors. The interference of the vortices may also cause an increase in noise. To address these issues, the vertical fin can be replaced by an annular airfoil (sometimes called a ring wing) having an inner diameter greater than the diameter of the tail rotor and which can be mounted around the tail rotor.